Polyimides (“PIs”) have excellent properties in comparison to other polymers. They have remarkable heat-resistant properties [1]. Also, PIs have exceptional mechanical strength, flexural modulus, toughness, chemical resistance, and good adhesion to various substrates [2]. In addition, they also have a low coefficient of thermal expansion, which allows them to be used in high-temperature applications [3]. These excellent properties of PIs can be attributed to the strong intermolecular forces between the polymer chains. Also, PIs have the ability to form hydrogen bonds due to partially positive hydrogen in the imide groups and partially negative oxygen on carbon double bonds, which will attract each other and form hydrogen bonds. Hydrogen bonds are stronger bonds than bonds from van der Waals forces. The phenylene ring structure also contributes to the strength and stiffness of PIs because the ring lacks the flexibility of single bonds. PIs are widely used in the electronic industry for various applications ranging from displays in computers and other digital devices to high-temperature adhesives in the semiconductor field and flexible insulated cables [4]. PIs are used to make mechanical and electrical parts in the automotive industry such as pumps, bearings, seals, valve seats, oil filter seals, retainer, piston rings, sensors/solenoids, relays, switches, ignition coils, etc. [5,6]. A more recent application of PIs is as substrates for solar cells and solar concentrators [7-15]. This is due to their good flexibility, mechanical strength, chemical resistance, and transparency.
Pristine PIs have many applications as mentioned previously. However, to serve in these applications more effectively, the properties of PIs need to be enhanced. Some of these applications include their use in electronics where the material needs to be electrically and thermally conductive. Other applications require PI with high strength, fracture resistance, creep resistance, and thermal stability, which the pristine material may not possess. These properties can be obtained by developing a nanostructured PI system. Nanostructured polymers are materials composed of a polymer in which nanoparticles (size ranging from 1 to 100 nm) [16] are incorporated to enhance the properties of the pristine polymer. One of the advantages of developing nanostructured polymers over the pristine polymer is that the properties of the nanostructured polymer can be tailored for specific applications. Other advantages include low cost, high strength, lightweight, ease of manufacturing compared to metals or ceramics, etc. In addition, small particles such as nanoparticles can induce crystallization by creating more crystallization sites than would larger particles [17]. An increase in crystallinity will increase properties, such as strength, modulus of elasticity, and thermal resistance. One important aspect that must be addressed when developing nanostructured polymeric systems is the choice of a suitable nano-reinforcement that is compatible with the polymer of choice. Additionally, the nano-reinforcement must possess excellent properties in order to improve the properties of the polymer with which it is combined.
Nanostructured polymeric systems and coatings have been recently investigated by the authors [18,19]. It was found that the corrosion resistance of poly(vinyl chloride-co-vinyl acetate) coatings have been significantly improved with the addition of nano-silicates [19]. Aglan et al. [20] also found that the corrosion resistance of the same coating material was enhanced with the addition of multi-walled carbon nanotubes (MWCNTs). Other properties such as the strength of the coating were also investigated and were improved with the incorporation of MWCNTs. It was also shown that tailoring certain properties can be done by using MWCNTs and nano-silicates [21,22]. Hedia et al. [21] found that 1% MWCNT reinforcement epoxy nano-adhesive had increased ultimate and residual strength of about 29% and 56%, respectively, when compared to the neat epoxy system. Also, in comparison with the neat resin, there was a 265% increase in the fracture toughness of the MWCNT adhesive.
The adhesive strength of thermoplastic PIs filled with MWCNTs and aluminum nitride nano-powder has been studied [23-25]. The authors found that there was a gradual increase in the tensile strength and modulus of the PI carbon nanotubes composite as the nanotubes loadings increased from 0 to 1.5 wt % loading. They also found that the tensile breaking energy increased with increase in nanotube loading up to 0.5 wt %. These improved properties were made possible by obtaining good dispersion achieved by mixing low percentage weight of MWCNT through mechanical agitation and sonication for long hours. However, it must be mentioned that the aspect ratio of the nanotubes is reduced during the sonication, even though improved dispersion is obtained. Further results showed that the failure mode of the adhesive joints changed from a mixed mode (adhesive/cohesive) for the neat sample to mostly adhesive failure as the MWCNT loading increased [24].
Other researchers also developed nanostructured PI with various nanoparticles and studied the effects of the nanoparticles on their properties [26-33]. The effects of carbon nanotubes on the mechanical and electrical properties of PI nano-composites were investigated [26]. It was reported that the strength of the PI increased with increasing loading of MWCNTs, while the strain to failure decreased, suggesting that the samples became more brittle with the addition of MWCNTs. Again the authors attributed this improvement in strength to the dispersion of the MWCNTs in the PI.
Processing conditions for nanostructured films include drying temperature and environment, method of dispersing the MWCNTs in the polymer systems, and method of fabricating the films. Additionally, the effects of various solvents on the mechanical performance of the films may be a concern in processing when solvents are used. Kobayashi et al. [34] studied the effect of thermal annealing on the conductivity of MWCNT-PI films prepared using an ultrasonic homogenizer and a spin coater. It was found that the resistance decreased with annealing. Various methods of dispersing MWCNTs into polymers have been explored over recent years [34, 35, 22], which include sonication, milling, surface modification of the MWCNTs to make them compatible with the polymer, melt processing, etc. It is therefore evident that processing conditions can greatly affect the mechanical performance of polymer films. The mechanical performance of the PI films in this work is characterized by ultimate strength, fracture resistance, and creep recovery. Creep recovery study is usually performed to investigate the deformation of a polymeric material with time because the strain of a viscoelastic material is time dependent. Creep strain is a barrier to some applications of thermoplastic polymers because the strain may accumulate and exceed the material's strain limit, leading to creep fracture. It is therefore important to study this property and ways of enhancing the creep resistance of the developed PI.
Thus, it is an aim of one or more inventions disclosed herein to provide methods for dispersing MWCNTs into PI that produce composite materials optimized to obtain maximum mechanical performance.
Additionally, it is an aim of one or more of the inventions disclosed herein to provide methods that utilize optimized processing conditions for neat and nanostructured PI films, including for example, solvent selection, drying environment, and temperature.
Also, it is an aim of one or more of the inventions disclosed herein to provide improved methods for formulating films neat and nanostructured PI films.
Furthermore, it is an aim of one or more of the inventions disclosed herein to provide neat and nanostructured films characterized by improved strength, fracture resistance, and creep recovery.